Usually, vehicles such as an Electric Vehicle (EV) and a Hybrid Vehicle (HV) obtain a drive power from an electric energy by converting a DC power supplied from a high-voltage battery into a three-phase AC power by an inverter and driving a three-phase AC motor by the three-phase AC power. Conversely, during a decelerating operation of the vehicle, a regenerative energy obtained by regenerative power generation of the three-phase AC motor is stored in the battery so that the driving is performed without wasting the energy.
In the above hybrid vehicles and electric vehicles, the inverter generates heat due to a switching operation of a switching element, and therefore may be thermally destroyed. For protecting the inverter from overheating, a cooling device is employed for the inverter (see, e.g., Japanese Patent Laying-Open Nos. 2003-018861, 2004-324613 and 11-294164).
Various cooling control devices for inverters have been studied. In an example, a temperature sensor is arranged near a cooler of an inverter, and a loss (heat value) of a switching element is arithmetically obtained from a temperature of the cooler sensed by the temperature sensor. A temperature of a joint portion where the switching element is joined to a substrate is arithmetically obtained based on the temperature of the cooler and the loss of the switching element, and the activation and deactivation of a water pump passing the cooling water through the cooler and on/off of a radiator fan for cooling the cooling water are controlled based on the temperature of the joint portion.
As another example, Japanese Patent Laying-Open No. 2003-018861 has disclosed an inverter cooling control device including cooling means that cools switching elements included in a plurality of inverters, respectively, temperature estimating means that estimates a temperature of a joint portion of the switching element of each inverter, and drive means that controls a cooling temperature of a coolant of the cooling means based on the estimated joint portion temperature.
More specifically, the temperature estimating means arithmetically obtains the joint portion temperature of the switching element of each inverter by thermally modeling the temperature of the inverter sensed by a temperature sensor, coolant and thermal resistances of the plurality of inverters. The drive means drives and controls a water pump and a radiator fan based on the arithmetically obtained joint portion to control the temperature of the coolant, and thereby controls the cooling capacity or power by controlling the coolant temperature. This structure can arithmetically obtain the joint portion temperature with precision. Therefore, an operation frequency of the drive means can be reduced and the power consumption can be suppressed, as compared with a conventional cooling control device of an inverter that cannot arithmetically obtains joint portion temperatures of a plurality of inverters with precision.
In the conventional cooling control device of the inverter described above, however, the estimation of the joint portion temperature of the switching element in the inverter can be performed with high precision, but requires complicated arithmetic processing. This results in a problem that the control of the cooling capacity of the cooler performed by the drive means cannot follow the actual rising of the joint portion temperature.
For example, when a motor drive current passing through the inverter rapidly increases in response to rapid variations in required motor output, a rising rate of the joint portion temperature is high so that it is necessary to increase rapidly the cooling capacity of the cooler. However, the cooler is controlled only after the temperature estimating means executes the estimation. Therefore, it is difficult to suppress the rising of the joint portion temperature, and may cause thermal destruction of the inverter.
For ensuring a control response of the cooler, such a manner may be employed that controls the cooler while fixing the cooling capacity at the value that is required when a thermal load attains the maximum, independently of the joint portion temperature.
However, this manner unnecessarily increases the power consumption of the cooler, and therefore may impair fuel efficiency of a vehicle equipped with the cooling control device of the inverter.
The invention has been made for overcoming the above problems, and an object of the invention is to provide a motor drive device that can achieve thermal protection of a drive circuit that drives and controls a motor as well as low electric power consumption of a cooling device.